Thromboembolic diseases remain the leading cause of death in developed countries despite the availability of anticoagulants such as warfarin (Coumadin®), heparin, low molecular weight heparins (LMWH), and synthetic pentasaccharides and antiplatelet agents such as aspirin and clopidogrel (Plavix®). The oral anticoagulant warfarin, inhibits the post-translational maturation of coagulation factors VII, IX, X and prothrombin, and has proven effective in both venous and arterial thrombosis. However, its usage is limited due to its narrow therapeutic index, slow onset of therapeutic effect, numerous dietary and drug interactions, and a need for monitoring and dose adjustment. Thus discovering and developing safe and efficacious oral anticoagulants for the prevention and treatment of a wide range of thromboembolic disorders has become increasingly important.
One approach is to inhibit thrombin generation by targeting the inhibition of coagulation factor VIIa (FVIIa). Factor VII is a plasma serine protease involved in the initiation of the coagulation cascade. It is present in human blood at a concentration of approximately 500 ng/mL, with about 1% of the total amount in the proteolytically active form factor VIIa (Morrissey, J. H. et al. Blood 1993, 81, 734-744). Factor VIIa binds with high affinity to its cofactor, tissue factor, in the presence of calcium ions to form a complex with enhanced proteolytic activity (Carson, S. D. and Brozna, J. P. Blood Coag. Fibrinol. 1993, 4, 281-292). Tissue factor is normally expressed in cells surrounding the vasculature, and is exposed to factor VIIa in blood by vessel injury or atherosclerotic plaque rupture. Once formed, the tissue factor/factor VIIa complex initiates blood coagulation by proteolytic cleavage of factor X to factor Xa, factor IX to factor IXa and autoactivation of additional factor VII to VIIa. Factor Xa, generated either directly by tissue factor/factor VIIa or indirectly through action of factor IXa, catalyzes the conversion of prothrombin to thrombin. Thrombin converts fibrinogen to fibrin, which polymerizes to form the structural framework of a blood clot, and activates platelets, which are a key cellular component of coagulation (Hoffman, M. Blood Reviews 2003, 17, S1-S5). In addition, there is evidence that tissue factor is present in blood, likely in an encrypted, form that is de-encrypted during clot formation. (Giesen, P. L. A. et al. Proc. Natl. Acad. Sci 1999, 96, 2311-2315; Himber, J. et al. J. Thromb. Haemost. 2003, 7, 889-895). The tissue factor/factor VIIa complex derived from blood borne tissue factor may play an important role in propagation of the coagulation cascade (clot growth) and in thrombus formation in the absence of vessel wall injury (i.e., stasis induced deep vein thrombosis or sepsis). The source of blood borne tissue factor is an area of active research (Morrissey, J. H. J. Thromb. Haemost. 2003, 1, 878-880). Therefore, factor VIIa plays a key role in propagating this amplification loop and is thus an attractive target for antithrombotic therapy.